solar design for a single communication tower with 34kwh consumption per day

[nirooafza]

I should install a single BTS communication tower on top of a mountain .... and i'm going to provide its power with solar panels .. . i have calculated the parts i need , but i think something is wrong with it .. . my system demands 34kwH per day and this communication tower consumes power 24 hours a day . the average sun hours is about 4.9 .. please some one help me with that ...

[Mike90250]

Move the tower to a place where it gets more sun ? (generally not practical)

You need more PV and battery to provide power 24/7 can you consider down time from midnight to 5am ? that will save power.

[Sunking]

Well I do this for a living for Verizon and ATT and your first mistake is using your average year Sun Hour Day which means your tower will be dark from about November to March. You have to use the shortest Sun Hour month, not yearly average.

At 34 Kwh/day for a Mission Critical system with 99.5% availability means your panels will need to generate a minimum of 57 Kwh/day of Power in the Month of December and January, and you will need a 25 Kva diesel generator with a tank large enough to last at least 3 days.

The equipment shelter will need to be much larger than normal to contain the massive amount of batteries required to do this. Normally with a AC powered system the batteries would be sized to support the site for 8 to 16 hours. That does not apply to a solar powered site. The batteries are going to need to be sized for 5 to 10 days. If you fail to do that you will be there once a year replacing batteries and suffering frequent power outages. So with 34 Kwh/day usage you are looking at a battery capacity of 170 to 340 Kwh or around 10,000 to 20,000 pounds worth.

As for panels you are going to be looking at about 4 to 6 separate groups of 5000 watt arrays each with its own charge controller sharing a common battery. Total wattage depends on your location. But assuming you get a 3.5 Sun Hour day in winter months you are looking at a 16,0000 to 18,000 watt solar panel array.

Now here is the really good news if you are an independent design/build contractor, you are going to make a lot of money. My firm charges T&E plus 15% and a project this size is going to cost the Telco around $160,000 to $200,000 or more if you will need a lifting Helicopter to get the equipment and materials up there. Helicopter charges are around $2000/hour.

[nirooafza]

thank you sunking for your help ... as you told, i'm an independent contractor .... money is not my problem ..... the problem is i should deliver a system that can work 24 hours a day and no interruption is acceptable for the local telecommunication office i'm going to work for.... with my calculations i would need 16400 AH battery bank ( sealed lead acid battery with 0.3 DOD and 95% efficiency) 40 solar panels (250W with Imp=8.2 & Isc=8.7) the total current for my charge controller is about 460 A ( MPPT charge controller for compensating energy losses during the hazy days) and a 2000W inverter ..... this calculations are for 2 days without sunshine ....
i think you are right i should change the average sunshine hour from 4.89 to minimum sunshine hour .

you mean 2 days without sun is not enough and i should consider it 5 to 10 days?
if i do this the battery bank would increase enormously ...
the 460 A for charge controller is very high ... there is no such charge controller that can deliver such current to me... even if i use MPPT modules which can be seried together i wont be able to pull such current

can i divide the system into 4 or 5 parts and connect the inverters to a single busbar?

[Sunking]

The problem is i should deliver a system that can work 24 hours a day and no interruption is acceptable for the local telecommunication office i'm going to work for....

Nirooafza I think I can safely assume you are not in the USA or a developed country because anything less than 24 hours per day power in the USA is unacceptable, so I am not sure how to respond to that question. That would be a question to ask the Telephone company you are working for and be put into the contract specifications. Obviously if the site is only operational say 12 hours per day completely changes the power requirements. However if only operational say 12 hours per day how you would automatically shut the site down so no power is consumed is not something I have ever heard of or done...

with my calculations i would need 16400 AH battery bank ( sealed lead acid battery with 0.3 DOD and 95% efficiency) 40 solar panels (250W with Imp=8.2 & Isc=8.7) the total current for my charge controller is about 460 A ( MPPT charge controller for compensating energy losses during the hazy days) and a 2000W inverter ..... this calculations are for 2 days without sunshine ....
i think you are right i should change the average sunshine hour from 4.89 to minimum sunshine hour .

What battery voltage are you planning on? Cell sites here in the USA operate at -48 volts DC, so a 5 day reserve for 34 Kwh/day is 170 Kwh and at 48 volts is 3500 Amp Hours. At 24 volts is 7000 AH

you mean 2 days without sun is not enough and i should consider it 5 to 10 days?
if i do this the battery bank would increase enormously ...
the 460 A for charge controller is very high ... there is no such charge controller that can deliver such current to me... even if i use MPPT modules which can be seried together i wont be able to pull such current
can i divide the system into 4 or 5 parts and connect the inverters to a single busbar?

Again the Telco makes that decision of how many days they want in reserve, but keep in mind you cannot fully discharge your batteries. Typically for a remote cell site you are going to use a VRLA AGM battery. I use a modular product from C&D VR Solar which you can see one of the cell sites I built in the picture. The batteries are in four cabinets sitting in front of the shelter. They are AGM batteries with built-in battery monitoring and management. At 20% DOD these batteries will yield up to about 6000 cycles. The key to how many days reserve is determined by the generator fuel capacity and your local climate. If the site is located where you get say 300 plus days of sunshine and easy access for fuel trucks you can cut the battery reserve to 3 days, but you are taking chances.

Ok for the charge controller you use multiple units. So lets say it will take a 16,000 watt solar panel array and the battery voltage is 48 volts. If it were me I would use 4-80 amp charge controllers. Each charge controller will have 4000 watt input from the panels, and all 4 CC feed a common battery bank or as we say the battery charge buss-bar

[nirooafza]

Again

Thank u Sunking .... I'm not form USA But I just said 24 hours a day to show that We can not stop energy production at all .
Of course, Its the telecommunication office that will be responsible for the lack of energy during cloudy days . because I will mention it in the contract .
My calculations are based on 24V . I used this voltage because i'm going to use MPPT charge controllers . With MPPT CC i can provide even more charging current during sunny or cloudy days . the Solar Panel Vmp is 30 V . So the 48V is not an Option .

But still I'm wondering how did you calculate the battery bank or charge controller current?
my battery bank is about 33200 AH & 12V which is equal with a 8300AH & 48V battery bank.
my calculations are for 2 days but your calculation for 5 days show lower amounts of battery bank.

my charge controller output current is 1440 A

I used the minimum sun hours during winter which is 1.59 Hours

[Sunking]

My calculations are based on 24V . I used this voltage because i'm going to use MPPT charge controllers . With MPPT CC i can provide even more charging current during sunny or cloudy days . the Solar Panel Vmp is 30 V . So the 48V is not an Option .

Hate to tell you this but 30 Vmp is not high enough to charge a 24 volt battery. It has to be a minimum of 36 volts and preferable much higher like up around 100 volts. It appears you do not know how a MPPT charge controller works. For example let's say I have a 100 watt panel rated 100 Vmp @ 1 Imp input to the Charge Controller connected to a 24 volt battery. The output to the battery is 24 volts @ 4.1 amps. A MPPT Charge controller is DC-to-DC voltage converter where it down converts voltage and up converts amperage much like an AC transformer will do. The whole purpose of using an MPPT controller is so you can use higher voltages on the input to lesson the current and gain efficiency.

But still I'm wondering how did you calculate the battery bank or charge controller current?
I used the minimum sun hours during winter which is 1.59 Hours.

That is real easy. You stated you need 34 Kwh per day right. So I calculated a 5 day reserve capacity so 5 x 34 Kwh = 170 Kwh. To find the Amp Hours needed factor out the battery voltage so at:

24 volts AH = Kwh / Battery Voltage = 170,000 = 24 = 7083 AH
48 volts AH = 170,000 / 48 = 3541 AH

For the Charge Controller is just as easy Amps = Solar Panel Wattage / Battery Voltage. Now that we have your real Sun Hours of 1.59 Hours things get really ugly because to generate 34 Kwh in winter you need a 32 Kw panel wattage. So at 24 volt battery the Charge Controller Current is 32,000 Kw / 24 volts = 1333 amps, or 666 amps at 48 volt battery. So if you use 80 amp CC go figure out how many you need.